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doi:10.2204/iodp.proc.344.103.2013 Structural geologyThe primary structural geology goal during Expedition 344 was to describe and document the style, geometry, and kinematics of structural features observed in the cores. Hole U1381C consists of cores from the sedimentary sequence (~105 m) and ends at the boundary to the underlying oceanic basement basalt of the Cocos plate (Table T2). Bedding was determined throughout the sedimentary sequence. The bedding dips vary from subhorizontal to gentle with a maximum value of 30° (Fig. F17). Sediment near the contact with the Cocos plate oceanic basement shows deformation bands, zeolite- and/or smectite-filled extensional fractures, and shear fractures. Structures within the sedimentary sequenceBedding dips were observed and measured in the sediment cores retrieved from Hole U1381C (Figs. F17, F18). Dip angles are generally subhorizontal in Unit I. In Unit II, a few dip angles vary between 20° and 31°. Bedding plane orientations were determined mostly on tephra layers, some of which show sharp scoured bases (Fig. F19). Erosional processes linked to the submarine flow of tephra are assumed to result in steeper than usual dip angles of bedding planes. Attitudes of bedding planes were restored to geographic coordinates using paleomagnetic data (Fig. F18). Restored bedding orientation data still show subhorizontal orientation without preferred attitude in Unit I. Unit II bedding planes preferentially dip gently either east or west. Distinct brittle deformation was observed in Unit III. Subhorizontal to steeply dipping deformation bands were observed in interval 344-U1381C-11H-5, 103–118 cm (Fig. F20). The deformation bands trend roughly ENE–WSW, subparallel to the trend of the Cocos Ridge (Fig. F21). Mineral-filled extensional fractures and shear fractures were observed in Sections 344-U1381C-11H-6 and 11H-7 (Fig. F20). Both types of fractures dip steeply (vertical to subvertical) and strike NNE–SSW. The shear fractures show a normal component of shear (Fig. F21). Displacement is at the scale of millimeters to >10 cm. |
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